1. Field of the Invention
The present invention relates to a nonvolatile memory using a semiconductor device.
2. Related Art Statement
Among memory devices using semiconductor devices, a memory device in which stored data is maintained without performing any special maintaining operation is called a nonvolatile memory.
A nonvolatile memory using a Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) has become much more important in accordance with a progress of the information-oriented society.
FIG. 1 is a cross sectional view showing a known floating gate type nonvolatile memory which has been used widely at present. A tunnel oxide film 106 is provided on a semiconductor substrate 101 in a surface of which are formed junction regions 102 and 103 for source electrode 104 and drain electrode 105, respectively, and a floating gate 107 made of a conductive polysilicon is formed on the tunnel oxide film 106. On the floating gate 107, an oxide film 108 and a control gate 109 are formed successively.
A substrate electrode 111 is provided to be connected to a rear surface of the semiconductor substrate 101, and a gate electrode 110 is formed to be connected to the control gate 109.
A voltage is applied across the substrate electrode 111 and the gate electrode 110 or across the substrate electrode 111 and the drain electrode 105 to inject carriers into the floating gate 107 through the tunnel oxide film 106 and the thus injected carriers are contained by the oxide film 108 between the control gate 109 and the floating gate 107.
When carriers are contained in the floating gate 107, a threshold voltage for a drain current assumes a first value, and when no carrier is contained, the threshold voltage assumed a second value which is different from said first value. In this manner, the memory device has two conditions in its characteristics. Then, information data can be stored memorized as these two conditions.
In the floating gate type nonvolatile memory explained above, it is important that carriers could be injected into the floating gate efficiently and the once injected carriers could be retained in the floating gate. By reducing a thickness of the tunnel oxide film 106, it is possible to raise the carrier injection efficiency, and the device property can be improved. However, this causes the deterioration of the tunnel oxide film, and the proper operation of the memory circuit might be lost. Therefore, a thickness of the tunnel oxide film could not be reduced beyond a certain limit in order to attain a reliability of the tunnel oxide film, and as a result, the operating voltage must be high for injecting a sufficient amount of carriers.
In order to retain the injected carriers, it is required that the oxide film 108 between the control gate 109 and the floating gate 107 has a certain thickness. It is apparent that the thick oxide film 108 might reduce the efficiency of the carrier injection into the floating gate 107 from the tunneling oxide film 106. In order to increase the carrier injection efficiency even increasing a thickness of the oxide film 108, it has been proposed to make a difference in a surface area between the control gate 109 and the floating gate 107. However, this solution make a structure of the memory device and a miniaturization of the memory device could not be attained.
Therefore, it is an object of the invention to provide a nonvolatile memory having a novel structure in which an operation voltage can be lowered, and miniaturization of the memory device can be realized easily.
According to the invention, a nonvolatile memory comprises a semiconductor substrate having at least one step formed in a first surface thereof; source and drain junction regions formed in the first surface of the semiconductor substrate on respective sides of said step; source and drain electrodes formed on said source and drain junction regions, respectively; an oxide film formed on the first of the semiconductor substrate to have a corner structure corresponding to said step of the semiconductor substrate; and a gate provided on said oxide film such that information is stored in the memory in accordance with a change in a threshold voltage due to carriers electrically injected into said gate film and captured therein.
As mentioned in H. J. Mattausch et al., Appl. Phys. Lett., vol. 71, p. 3391 (1997), it has been known that a stable carrier capturing is performed at the corner structure in comparison with the plane part. Such a phenomenon is called a deep level capture. This deep level capture is very stable, and even if a heating treatment at a higher temperature is performed, the capture of carriers at the corner structure is stable.
In the nonvolatile memory according to the invention, carriers are injected into the oxide film by an application of an electric field across the semiconductor substrate and the gate or across the gate and the drain. Though a part of the injected carriers flows into the gate electrode, a remaining part of the injected carriers is captured in the oxide film stably to change a threshold voltage for the drain current.
In the known floating gate type nonvolatile memory, carriers are injected into the floating gate through the tunnel oxide film and are contained therein. Contrary to this, in the memory according to the invention, carriers are captured in the oxide film. Therefore, the memory can be operated at a lower voltage than the conventional floating gate type nonvolatile memory.
Furthermore, according to the invention, it is no more necessary to provide the oxide film for containing carriers, and thus the memory has a very simple structure. Therefore, the manufacturing cost of the nonvolatile memory according to the invention can be reduced greatly.
An optimum operating voltage of the nonvolatile memory according to this invention is determined by a thickness of the oxide film. In other words, a thickness of the oxide film can be determined by a desired operating voltage. It should be noted that when the oxide film is thinner than 10 nm, the deep level capture of carriers is decreased abruptly. Therefore, it is preferable that a thickness of the oxide film should not be less than 10 nm.
In an embodiment of the nonvolatile memory according to the invention, a thickness of the oxide film is not less than 10 nm.
As mentioned above, in the conventional floating gate type nonvolatile memory, the tunnel oxide file must be made thin in order to improve the device property. In the nonvolatile memory according to the invention, since carriers are captured in the oxide film, if the oxide film is too thin, carriers could not be effectively captured by the oxide film and the memory device could not work satisfactorily. Therefore, the nonvolatile memory according to the invention can be manufactured easily, precisely and reliably by means of the well developed manufacturing technology, and the manufacturing cost can be reduced.
In the memory according to the invention, the drain junction region is formed to extend under the oxide film. In this case, a distance from the corner to an inner edge of the drain junction region is determined in accordance with a channel length and a desired property of MOSFET. Particularly, it is preferred to extend the drain junction region under the oxide film such that the inner edge of this region is apart from the corner not less than 0.1 xcexcm.